$\forall$${\it es}$:ES, $e$, ${\it e'}$:E. ($\uparrow$first($e$)) $\Rightarrow$ ($\uparrow$first(${\it e'}$)) $\Rightarrow$ (loc($e$) = loc(${\it e'}$) $\in$ Id) $\Rightarrow$ ($e$ = ${\it e'}$)